Although many studies have been made on the nonmagnetic heavy fermion CePtSi belonging to the o.-ThSi2 structure, to the best of our knowledge no studies have been reported in the other rare-earth members of this series. In this paper, we report our resistivity, magnetization, and heat capacity studies on NdPtSi and SmPtSi. We have established bulk antiferromagnetic ordering in NdPtSi below 3.8 K and ferromagnetic ordering in SmPtSi below 15 K. The heat-capacity studies show large contribution from the crystal-field effects. We have also established bulk superconductivity below 3.8 K in LaPtSi from resistivity, susceptibility, and heat capacity studies. A detailed study

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Positive giant magnetoresistance (GMR), Δρ/ρ, is reported here in polycrystalline antiferromagnetic materials RE2Ni3Si5 (RE=Tb, Sm, Nd); Δρ/ρ, at 4.4 K and in a field of 45 kOe, is 85%, 75%, and 58%, respectively. Positive GMR of such large magnitude has not been reported earlier in magnetically ordered polycrystalline compounds. The observed GMR is not correlated to the RE-moments. It is, however, associated with the magnetic ordering of the lattice as its magnitude is significantly reduced in the paramagnetic state. Surprisingly, MR in Y2Ni3Si5, a non-magnetic rare earth analogue, is also relatively large (16% at 4.4 K; 45 kOe) and is even slightly higher than that of antiferromagnetically ordered Gd2Ni3Si5 (12% at 4.4 K; 45 kOe). The layered structure of the materials is suggested to be responsible for the observed GMR.